The present invention relates generally to rigid mounts for expansible bodies. More particularly, the invention relates to a rigid mount for an internally pressurizable plastic tube which does not restrict radial expansion of the tube due to internal pressure.
The mounting of glass reinforced plastic (GRP) gun tubes, recoilless rifles or rocket launchers to a stable launch platform has always been a problem because of the low stiffness, high radial expansion and weak interlaminar shear strength of this material. The mounts used for these devices must however be rigid in order to obtain a desired degree of accuracy of fire. Prior art metal mounting rings of the type generally shown in U.S. Pat. No. 2,969,214, issued Jan. 24, 1961 to Torresen and U.S. Pat. No. 2,684,221, issued July 20, 1954 to Wollam have been bonded or mechanically attached to the outside diameter of the GRP tube thus forming a convenient attachment point. In use, the metal ring or mount could be welded to or threaded to a tripod, aircraft munitions rack or other appropriate structural member in order to provide a stable launch platform. Welding directly to the plastic tube is not a viable or convenient mode of attachment because of the characteristics of the material. Also, bolting or threading the mount directly to the GRP tube is not advisable because of the high stress concentrations invariably induced in the GRP tube. Most importantly the major disadvantage of a metal mounting ring is that it constrains or restricts the radial expansion of these pressurizable bodies or expansible tubes when internal pressure is present within the tube, e.g. at launch. This restriction results in relatively high bending stresses or discontinuity stresses in the GRP tube where the edge of the metal ring contacts the tube.
In order to partially overcome the severe restrictions of a metal ring used by the prior art, an attachment method which has been frequently used consists in bonding a plurality of metal flanges about certain portions of the periphery of the tube and constraining the extremities of the flanges by GRP strapping bands which are also bonded to the tube. The flanges may have mounting ears which can be welded or mechanically attached to a launch platform. However, this method has also shown to be unsatisfactory because radial restriction occurs at the ear locations on the flange thus inducing bending stresses in the tube. Furthermore, a transverse load imposed on the tube will cause concentrated bending stresses in the tube proximate the ear locations.
Alternatives to using a metal ring or flanges with ear attachments, both of which have been shown to induce discontinuity stresses in pressurizable plastic tubes include the following: (1) increasing the thickness of the tube locally with GRP materials and mechanically attaching a mounting surface to this increased diameter section and (2) attaching the mounting surface directly to the tube wall. The former method unfortunately and unduly restricts radial expansion, albeit not as much as for a metallic ring. The latter method is also undesirable in that it may require attachment holes or slots formed in the tube wall which severly weaken relatively brittle materials like GRP.